A nonaqueous secondary battery having a high electromotive force that comprises a nonaqueous electrolyte and utilizes oxidation and reduction of lithium has recently been used as one of new type high power and high energy batteries.
In such a nonaqueous electrolyte secondary battery, a lithium-transition metal composite oxide which lithium ions can be intercalated into and deintercalated from can be used as a positive electrode active material for a positive electrode. A lithium-cobalt composite oxide, LiCoO2, is generally used.
However, cobalt, a material of a lithium-cobalt composite oxide, is expensive. Furthermore, in a nonaqueous electrode secondary battery comprising the lithium-cobalt composite oxide as a positive electrode active material, when lithium ions are deintercalated from the lithium-cobalt composite oxide an oxidation number of cobalt in the composite oxide becomes high, and the composite oxide reacts with the nonaqueous electrolyte. The reaction is vigorous when a temperature is high and causes various problems, for example, battery capacity of the nonaqueous electrolyte secondary battery is deteriorated.
A lithium-transition metal composite oxide containing a transition metal besides cobalt, for example, nickel, manganese, and the like, has been considered.
In Japanese Patent Nos. 2561556 and 3244314, a nonaqueous electrolyte secondary battery in which a lithium-transition metal composite oxide containing cobalt, nickel and manganese as a positive electrode active material is proposed. There is a report in Electrochemical and Solid-State Letters, 4(12)A200-A203 (2001) that if a lithium-transition metal composite oxide containing cobalt, nickel and manganese represented by LiMnxNixCo1-2xO2, in which the mole ratios of manganese and nickel are the same, is used, reaction of the composite oxide with the nonaqueous electrolyte at a status of charge is inhibited and the battery has excellent heat stability.
However, when the lithium-transition metal composite oxide represented by the formula described above that contains manganese and nickel in equal mole ratios is used for a positive electrode active material, discharge characteristics of the positive electrode are deteriorated, especially when a battery is discharged at a large current discharge capacity is significantly reduced, and there is a problem that high rate discharge characteristics are not good.
To improve discharge characteristics of a positive electrode, Japanese Patent Laid-open Publication No. 11-176446 proposes to add a binder of a mixture of fibrillar carbon and granular carbon to a positive electrode active material. However, even when such binder is added, high rate discharge characteristics cannot be improved, sufficiently.